1. Field of the Invention
The present invention relates to a bearing steel part for a rolling bearing and more particularly to a bearing steel part for a rolling bearing used for high speed operation in a gas turbine engine, a machine tool and so on.
2. Description of the Background Art
AISI M50 which is highly heat-resistant Cr--Mo--V based high speed tool steel and its improvement M50 NiL have been used as steels of bearing parts, for example, for gas turbine engines. However, the development of material having higher fracture toughness is currently required for higher speed.
In short, for higher speed, a dn value (a product of the inner diameter d of a bearing part and its rotational speed n), an index indicating the heaviness of a load on the bearing part, is larger, and the bearing part should resist to the heavier load. The larger dn value causes larger tensile stress to the bearing part. If a crack is caused, a crack growth rate may be increased and it may lead to a damage of the bearing. Therefore, material having higher fracture toughness should be developed in order to suppress the crack growth.
For this purpose, Japanese Patent Laying-Open No. 7-252598 discloses martensite-based steel of the Fe group containing at most 0.4 wt. % of C (carbon), 2-7 wt. % of Cr (chromium), 3-20 wt. %, as W equivalent weight (W+2Mo), of one or two of W (tungsten) and Mo (molybdenum), and 0.5-1.1 wt. % of V (vanadium). With this composition, this martensite-based steel improves the fracture toughness value of the bearing part without lowering its hardness.
However, the hoop stress of at least 400 MPa is imposed on a bearing used at the dn value of at least 4 million when the bearing is rotated. Although the fracture toughness value of at least 80 MPam.sup.1/2 is necessary to prevent crack growth against this hoop stress, such a high fracture toughness value cannot be achieved with the existing material as above.
There is a great demand for higher speed in machine tools as well. The dn value of 3.5 million has been achieved in angular ball bearings by a lubrication method combining under-race lubrication and jet lubrication. However, development of a bearing which can be used at the dn value of 4 million is desired. In order to achieve this, material having the aforementioned fracture toughness value should be developed.
Higher durability as a bearing, that is, higher rolling contact fatigue strength is also important especially for gas turbine engines. For improving the rolling contact fatigue strength, Japanese Patent Laying-Open No. 62-132031 discloses increasing the core hardness of bearing steel, or carburized steel in this case, to at least HRC 48. On the other hand, decreasing the core hardness has been known as a way of improving the fracture toughness value of the carburized steel. In short, when the core hardness of the bearing part is increased to improve its rolling contact fatigue strength, the fracture toughness value of the bearing part is lowered. Thus, the fracture toughness value and the hardness are incompatible characteristics. In order to obtain a sufficiently superior rolling contact fatigue life characteristic under the condition of the higher dn value, it is desirable to enhance the fracture toughness and the hardness without sacrificing either of these characteristics.